Generally, a nitrogen oxide purification catalyst is disposed on an exhaust line to reduce nitrogen oxide (NOx) of exhaust gas, and alkali metals or alkali earth metals are formed therein together with a noble metal.
The nitrogen oxide purification catalyst absorbs NOx in a lean condition of exhaust gas, discharges NOx in a rich condition of exhaust gas, and reduces the discharged NOx.
The intake air amount can be reduced or the exhaust gas can be recirculated so as to regenerate or desulfurize the nitrogen oxide purification catalyst. The lean/rich condition of exhaust gas can be controlled by controlling the injection timing or the injection amount of fuel that is injected into a cylinder.
A lambda sensor (or oxygen sensor) is disposed on an exhaust line so as to control the lean/rich condition of exhaust gas, and the lean/rich condition of exhaust gas is detected through a lambda signal that is transmitted from the sensor.
A rich mode is performed to control the exhaust gas to the rich condition so as to reduce the NOx that is absorbed in the nitrogen oxide purification catalyst while the rich mode is performed, and to reduce oxygen that is stored in the nitrogen oxide purification catalyst. A diesel particulate filter is decreased, and the nitrogen oxide that is absorbed in the nitrogen oxide purification catalyst is released to be reduced.
Hydrocarbon and carbon monoxide that are included in the exhaust gas are not oxidized at an end stage of the rich mode, the amount that is exhausted to an outside is increased, and the rich mode is not sufficiently continued by the lambda signal that is generated from the lambda sensor.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.